Monday
April 30, 20074:00 PM,
5130 Upson Hall

Abstract:

We consider the problem of ranking or clustering a set of
elements, based on input information for each pair of elements. The objective is
to find a solution that minimizes the deviation from the input information. For
example, we may want to cluster webpages based on similarity scores, where for
each pair of pages we have a score between 0 and 1, and we want to find a
clustering that minimizes the sum of the similarity scores of pages in different
clusters plus the sum of (one minus the similarity score) for pages in the same
cluster. Another example arises in meta-search engines for Web search, where we
want to get robust rankings that are not sensitive to the various shortcomings
and biases of individual search engines by combining the rankings of the
individual search engines.

Randomized approximation algorithms for unconstrained versions
of these problems were given by Ailon, Charikar, and Newman, and by Ailon and
Charikar. We give deterministic approximation algorithms for these problems,
thereby answering an open question of Ailon et al.

Our algorithms follow the paradigm of Ailon et al. of choosing
a particular vertex as a pivot and partitioning the graph according to the
pivot; unlike their algorithms, we do not choose the pivot randomly.

Additionally, our algorithms extend to constrained versions of
these problems in which we have apriori constraints on the output. For instance
for the ranking problem, we can have as input a partial order such that the
output ordering must be a linear extension of this ordering.

This is joint work with Rajneesh Hegde, Kamal Jain and David
P. Williamson.